To prevent counterfeiting and to enable authenticity to be checked, security sheets such as passport security pages, driving licenses and identify cards are typically provided with one or more security devices which are difficult or impossible to replicate with commonly available means such as photocopiers, scanners or commercial printers.
One well known type of security device is one which uses a colour shifting element to produce an optically variable effect that is difficult to counterfeit. Such a colour shifting element generates a coloured appearance which changes dependent on the viewing angle. Examples of known colour shifting structures include photonic crystals, liquid crystals, interference pigments, pearlescent pigments, structured interference materials or thin film interference structures including Bragg stacks.
It is also known in the art that the optical effect produced by a colour shifting element can be modified by introducing a film comprising a surface relief over the colour shifting element, wherein the surface relief modifies the angle of light incident to, and reflected from, the colour shifting element so as to provide a different optical effect to the viewer. For example, such an additional “light control” layer may produce colour shifting effects which are visible closer to a normal angle of viewing with respect to the device, and may enable more colours to be viewed on tilting the device as compared to the colour shifting element in isolation. WO2009/066048 describes the combination of a colour shifting element with such a “light control” layer.
The optical effect provided by a combination of a colour shifting element and such a “light control” layer enables a security device to be created that exhibits a memorable effect to a viewer that is easy to authenticate and yet difficult to counterfeit. Such a security device may be incorporated into a security sheet, typically by adhering the security device to an outer layer of the security sheet or by partially embedding the security device as a security thread within the security sheet in order to increase the security of the security sheet.
However, the process of manufacturing a plurality of such security devices to then incorporate onto a security sheet is time consuming and inefficient. Furthermore, by adhering the security device to a security sheet, the security device is vulnerable to damage or tampering.